In the petroleum drilling process, the encountered formations are more and more complex, and the numbers of exceptional wells, ultra-deep wells, and complex wells increase with years, which brings higher requirements for the drilling fluid. In view of that situation, domestic and foreign researchers have made a series of researches on high temperature resistant filtrate reducers for drilling fluid. However, for existing filtrate reducers for drilling fluid, the dosage of the filtrate reducer for drilling fluid must be increased to improve high temperature resistance and salt resistance performance of deep wells and ultra-deep wells. But increased dosage of filtrate reducer for drilling fluid will cause increased viscosity of the drilling fluid; especially, in the case of high calcium content and/or magnesium content, it is more difficult to control the properties of drilling fluid. Therefore, existing filtrate reducers for drilling fluid can't fully meet the requirements for well drilling. Shale gas is exploited mainly in shallow extended reach wells and clustered horizontal wells, owing to the accumulation characteristics of shale gas. Owing to the fact that the water sensitivity of fractures in shale formations is high and shale formations usually have long horizontal sections, well leakage, collapse, and shrinkage phenomena may occur during well drilling in the long horizontal sections, and friction resistance, rock carrying, and formation contamination problems may exist, increasing complexity in the well environment. Therefore, in drilling fluid selection and design, it is crucial to ensure well wall stability, reduce resistance and friction, and remove cuttings beds in horizontal well drilling for shale gas. To meet the requirement for well wall stability, oil-based drilling fluid systems are used in 60-70% horizontal shale gas wells in foreign countries; in addition, oil-based drilling fluids are used in most horizontal shale gas wells finished recently in China. However, water-based drilling fluids are the best choice, because oil-based drilling fluids have high cost and environmental pollution problems. Nevertheless, researches have shown that the active shale rocks may have water loss or dehydration phenomenon when they are exposed to water-based drilling fluids with high salinity (e.g., 35% CaCl2 water solution). Hence, it is urgent task to develop a filtrate reducer that has outstanding filtrate reduction performance and viscosity increasing effect, to meet the requirements of horizontal well drilling for shale gas.